(107f) Multinuclear, Multidimensional, and Multi-Field NMR Reveals Complex Al and OH Site Distributions in Zeolite Catalysts

Zeolites are interesting materials from both practical catalysis applications and fundamental structure-function perspectives. With known uses ranging from cracking, isomerization, and methanol-to-hydrocarbon chemistries, the traditional view of only one type of acidic Brønsted proton site in many zeolite catalysts has been called into question by recent experimental NMR data on zeolites with relatively high framework Al content. Motivated by surprising increases in reaction rates observed for some hydrocarbon/HZSM-5 probe reactions in the presence of sub-stoichiometric amounts of water, a series of advanced solid-state NMR experiments reveals that more than one framework Al site as well as associated Brønsted acid protons can exist in zeolite HZSM-5. These sites are best described as fully-coordinated framework and partially-coordinated framework Al sites. The number and type of Al-OH sites can be varied, with significant impacts on reactivity even in cases when no extra-framework Al species can be detected. To date, experiments suggest that proximity between the two type of framework Al sites and their respective OH groups is important for reactivity. When such species are selectively removed, reaction rates decrease by an order of magnitude or more as first revealed by in-situ MAS NMR and confirmed by reactor studies, and addition of any amount of water only leads to decreases in reaction rates.